[AGP 9] Added Proguard File Check (flutter#178191)

Starting AGP 9 the `android.proguard.failOnMissingFiles` flag is set to
true by default
[here](https://developer.android.com/build/releases/agp-preview#:~:text=change%20is%20required.-,android.proguard.failOnMissingFiles,-Fails%20the%20build).
This means that specified proguard files must exist in order for the
proguard rules to be configured. Added a check to ensure the file
exists.

Addresses flutter#175688

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Author: jesswrd

packages/flutter_tools/gradle/src/main/kotlin/FlutterPlugin.kt

@@ -265,15 +265,18 @@ class FlutterPlugin : Plugin<Project> {
                     // Enables resource shrinking, which is performed by the Android Gradle plugin.
                     // The resource shrinker can't be used for libraries.
                     isShrinkResources = FlutterPluginUtils.isBuiltAsApp(project)
-                    // Fallback to `android/app/proguard-rules.pro`.
-                    // This way, custom Proguard rules can be configured as needed.
                     proguardFiles(
                         FlutterPluginUtils
                             .getAndroidExtension(project)
                             .getDefaultProguardFile("proguard-android-optimize.txt"),
-                        flutterProguardRules,
-                        "proguard-rules.pro"
+                        flutterProguardRules
                     )
+
+                    // Optionally adds custom Proguard rules as needed from `android/app/proguard-rules.pro`.
+                    // Starting AGP 9.0 Proguard files must exist to be added to the configuration.
+                    if (File("${project.projectDir}/proguard-rules.pro").exists()) {
+                        proguardFile("proguard-rules.pro")
+                    }
                 }
             }
         }